X-rays being irradiated to a patient by an X-ray CT apparatus that reconstructs image data from the penetration data is known. With a multi-slice X-ray CT apparatus, it is possible to collect two or more slices of data at once from different positions (multi-slice scanning), using the X-ray detector which has two or more segments of detection elements, such as a combination of a scintillator and a photodiode, which detects the X-rays. By using the multi-slice scan (it is also called a cone beam scan) together with a helical scan, data from a large scanning range can be collected in short time, and the multi-slice X-ray CT apparatus has become popular.
When using the multi-slice scan together with a helical scan, an important issue is the reduction of the X-ray dose. For example, Japanese Patent Publication (Kokai) No. 2002-17716 and Japanese Patent Publication (Kokai) No. 10-248835 describe that a scanning range is set on a scanogram to include an object, such as an internal organ of a patient, an opening of a collimator is set according to the scanning range and a scan is performed of the internal organ. However, in fact, a part of the internal organ may not be included in the scanning range. As a result, insufficient data may be collected and a repeated scan may be necessary.